A field-based spectrophotometric flow-injection system for automatic determination of chloride in soil water

The use of chloride as a tracer for soil water investigations is discussed. Limitations with laboratory based soil core and field based sampling are considered with respect to the poor suitability of the data for rigorous assessment of mechanistic models. Investigation of water movement in soil has been restricted by limitations in spatial and temporal sampling. Fine resolution sampling generates large numbers of samples which cause problems with post sampling laboratory analysis. This paper describes a field-based system for the analysis of chloride in soil water. There are three component parts to the system, (i) a custom sampling sub-system comprising of ceramic cup suction samplers and sample traps, (ii) a sample routing sub-system to channel sample to (iii) a sample analysis sub-system utilizing a flow injection method for sample analysis using a custom built photo-diode detector. The three sub-systems were controlled by a suitably equipped personal computer. A calibration procedure is described with a third order polynomial equation derived to convert millivolt response from the detector into chloride concentration. Field and laboratory data from a tracer experiment are presented and discussed, and it is concluded that the system is well suited to field-based applications. Finally it is noted that the photo-detector is suitable for colourimetric analysis of any tracer with suitable chemical determination.     

Pilot study on the contamination of drinking water by organotin compounds from PVC materials

Raw and treated water samples and tap water samples from four to six houses located on distribution lines where PVC pipe/tubing had been recently installed were collected in five Canadian municipalities for the analysis of organotin compounds. After derivatisation with sodium tetraethylborate the ethylated organotin compounds were extracted with hexane and analysed by gas chromatography-microwave induced plasma atomic emission spectrometry using a wavelength (326.234 nm) specific for tin. Organotin compounds, mainly methyltin and dimethyltin at concentrations ranging respectively from 0.5 to 257 ng Sn/L and from 0.5 to 6.5 ng Sn/L, were detected in samples from ten of the twenty-two houses. No organotin compounds were detected in raw water or treated water leaving the treatment plant, indicating that the organotin compounds were leaching into the water from some component of the distribution system.     

Yard waste compost as a stormwater protection treatment for construction sites.

Runoff water quality improvement from three yard waste compost erosion control treatments were compared with two conventional treatments and an untreated control on plots of 3:1 slope during two growing seasons, using natural events and simulated rainfall. Runoff volume, suspended solids, nutrients, biomass, turf shear strength, and turfgrass color scale were monitored. The most effective compost treatment, a 5-cm thick blown compost blanket, produced 12.7 times less runoff and 9.8 times less sediment load than a straw mat and silt fence treatment. The compost treatments generated eight times more biomass than the straw mat treatments. Root development was significantly better on the compost treatments based on turf shear strength measurements. Tilled-in compost was not as effective as a compost blanket at reducing sediment loss, particularly before the establishment of grass on the plot. The cost of compost treatments was similar to that of straw mat with silt fence treatments.     

Studies on plant-mediated fate of the explosives RDX and HMX

The fate of the explosives RDX and HMX on exposure to plants was investigated in 'natural' aquatic systems of Myriophyllum aquaticum for 16 days, and in axenic hairy root cultures of Catharanthus roseus for > or = 9 weeks. Exposure levels were: HMX, 5 mg/l; and RDX, approximately 8 mg/l. Exposure outcomes observed include: HMX, no transformation by aquatic plants, and minimal biological activity by axenic roots; and RDX, removal by both plant systems. In the case of RDX exposure to axenic roots, since 14C-RDX was included, removal was confirmed by the accumulation of 14C-label in the biomass. The intracellular 14C-label in these RDX studies was detected in two forms: intact RDX and bound unknown(s).     

Variable-density groundwater flow and solute transport in heterogeneous porous media: approaches, resolutions and future challenges

In certain hydrogeological situations, fluid density variations occur because of changes in the solute or colloidal concentration, temperature, and pressure of the groundwater. These include seawater intrusion, high-level radioactive waste disposal, groundwater contamination, and geothermal energy production. When the density of the invading fluid is greater than that of the ambient one, density-driven free convection can lead to transport of heat and solutes over larger spatial scales and significantly shorter time scales than compared with diffusion alone. Beginning with the work of Lord Rayleigh in 1916, thermal and solute instabilities in homogeneous media have been studied in detail for almost a century. Recently, these theoretical and experimental studies have been applied in the study of groundwater phenomena, where the assumptions of homogeneity and isotropy rarely, if ever, apply. The critical role that heterogeneity plays in the onset as well as the growth and/or decay of convective motion is discussed by way of a review of pertinent literature and numerical simulations performed using a variable-density flow and solute transport numerical code. Different styles of heterogeneity are considered and range from continuously "trending" heterogeneity (sinusoidal and stochastic permeability distributions) to discretely fractured geologic media. Results indicate that both the onset of instabilities and their subsequent growth and decay are intimately related to the structure and variance of the permeability field. While disordered heterogeneity tends to dissipate convection through dispersive mixing, an ordered heterogeneity (e.g., sets of vertical fractures) allows instabilities to propagate at modest combinations of fracture aperture and separation distances. Despite a clearer understanding of the processes that control the onset and propagation of instabilities, resultant plume patterns and their migration rates and pathways do not appear amenable to prediction at present. The classical Rayleigh number used to predict the occurrence of instabilities fails, in most cases, when heterogeneous conditions prevail. The incorporation of key characteristics of the heterogeneous permeability field into relevant stability criteria and numerical models remains a challenge for future research.     

Biogeochemical characterisation of a coal tar distillate plume

The distillation of acidified coal tars for up to 50 years has given rise to a phenol plume approximately 500 m long, 50 m deep and containing up to 15 g l(-1) dissolved organic carbon (DOC) in the Triassic Sandstones aquifer. A conceptual biogeochemical model based on chemical and microbiological analysis of groundwater samples has been developed as a preliminary to more detailed studies of the controls on natural attenuation. While the development of redox zones and the production of methane and carbon dioxide provide evidence of natural attenuation, it appears that degradation is slow. The existence of sulphate in the plume indicates that this electron acceptor has not been depleted and that consequently methanogenesis is probably limited. Based on a simple estimate of sulphate input concentration, a half-life of about 15 years has been estimated for sulphate reduction. Geochemical modelling predicts that increased alkalinity within the plume has not led to carbonate precipitation, and thus within the limits of accuracy of the measurement, alkalinity may reflect the degree of biodegradation. This implies a loss of around 18% of the DOC over a 30-year period. Despite limited degradation, microbial studies show that there are diverse microbial communities in the aquifer with the potential for both anaerobic and aerobic biodegradation. Microbial activity was found to be greatest at the leading edge of the plume where DOC concentrations are 60 mg l(-1) or less, but activity could still be observed in more contaminated samples even though cells could not be cultured. The study suggests that degradation may be limited by the high phenol concentrations within the core of the plume, but that once diluted by dispersion, natural attenuation may proceed. More detailed studies to confirm these initial findings are identified and form the basis of associated papers.     

Large-scale baseload wind power in China

This paper presents a novel strategy for developing wind power in large-scale (multi-GW) wind farms in China. It involves combining oversized wind farms, large-scale electrical storage and long-distance transmission lines to deliver 'baseload wind power' to distant electricity demand centers. Baseload wind power is typically more valuable to the electric utility than intermittent wind power, so that storage can be economically attractive even in instances where the cost per kWh is somewhat higher than without storage.
The prospective costs for this approach to developing wind power are illustrated by modifying an oversized wind farm at Huitengxile, Inner Mongolia. The site has an average power density of 580 W/m² at 50 m hub heights and is located 500 km north of Beijing. Using locally mass-produced wind turbines there are good prospects that wind power would be cost-competitive with coal power, on a lifecycle cost basis, while providing substantial net environmental benefits.
Finally, the institutional challenges related to the prospect of large-scale wind energy development are addressed. Especially important are policies aimed at developing the capacity for mass production of as much of this technology in China as is feasible. Promising instruments for speeding up the introduction of this technology include: (i) international joint ventures between foreign vendors and developers and Chinese manufacturers; and (ii) wind resource development concessions.